1. Field of the Invention
The present invention relates to an image forming apparatus. Particularly, it relates to a two-color image forming apparatus for forming a simultaneous two-color image with plural colors, more specifically, to improvement of a mixed-color separation technique at development in a two-color printer and a two-color copying machine, having a two-color image forming process based on an electro-photographic process.
2. Description of the Related Art
In a simultaneous two-color copying machine, for example, by a black toner and a red toner (color toner) as a two-color image forming apparatus, a block image and a red image are read by a light receiving element, and by laser beam having two kinds of wave-length modulated respectively responsive thereto, two different locations in a circumferential direction on a photosensitive drum are exposed to form latent images of each of the respective color images.
At this time, in order to improve visual recognition of the image, the exposed positions of respective colors are such that, usually a red image is charged and exposed on the more upstream side in the rotating direction of the photosensitive drum than a black image, and is developed by a first developing unit containing the red toner. The black image is recharged and exposed after development of the red image, and developed by a second developing unit containing the black toner.
Accordingly, there is a possibility that the red toner is contacted to a developing agent in the second developing unit and scraped off and mixed therein, when the red image having developed by the first developing unit passes through the position of the second developing unit, thereby when the black image is developed, there is a possibility that the image which is originally black may change into a mixed-color image of black and red.
Such problem is encountered conspicuously in the case where the second developing unit is a so-called magnetic brush-type developing unit. This is because, in the magnetic brush-type developing unit, the development takes place by contacting a magnetic brush of the developing agent composed of toner and carrier to the photosensitive drum, so that the red toner which has been developed previously by the first developing unit is scraped off by the magnetic brush formed of the second developing unit.
In order to prevent a so-called color mixture as stated above, conventionally, a technique disclosed, for example, in the U.S. Pat. No. 4,822,702 is known as a mixed-color separation technique.
In the aforesaid U.S. Pat. No. 4,822,702, a two-component developing agent composed of the red toner and carrier is contained in the first developing unit located on the up-stream side of the photosensitive drum, and a two-component developing agent composed of the black toner and carrier is contained in the second developing unit located on the downstream side thereof. Both the red and black toners used here are the non-magnetic toner. The red toner has such a characteristic that it is charged in opposite polarity by a friction charge when mixed with a second developing agent containing the black toner. That is, there is a difference in physical properties between the red toner and the black toner.
Since the red toner having the aforesaid characteristic is stuck to a non-image portion (charged portion) of the photosensitive drum at development by the second developing unit when mixed with the second developing agent, it is removed from the second developing unit. Since a charge polarity of the red toner stuck to the non-image portion on the photosensitive drum is the same polarity as an applied voltage of a transfer charger, the red toner remains on the photosensitive drum and is scraped off and recovered by a cleaner without being transferred onto a copying paper. In the U.S. Pat. No. 4,822,702 stated above, mixed-color separation is effected automatically during development in such a manner.
As disclosed in Japanese Patent Application Laid-Open No. 58-102251, a technique for separating the first toner by giving a difference between development thresholds of the first toner and the second toner, or in other words, giving a difference between development starting potentials by the toners, and using a recovery roll applied with a bias voltage is known.
In the prior art aforementioned, however, though it is theoretically possible to separate only the first toner from the second developing agent, in practice, a considerable amount of second toner is also contained in the first toner being recovered, thus it is impossible to separate completely. In the case where the first toner is mixed only a little into the second developing agent, most of the toner being recovered is the second toner, result in waste of the second toner.
It is believed that such a problem can be solved to some extent by setting the conditions of separating the first toner severely. In that case, however, there is a high possibility that such conditions are not met, because kinds and ranges of the toner which can be used are limited, and charging characteristics of the toner changes due to variations in the surrounding environment, so that it is not an effective way.
Furthermore, even when a difference of physical properties exists between the toners of different colors as stated above, when the amount of red toner mixed into the second developing agent exceeds the separated amount, a mixed-color separation may not be effected sufficiently only by the mixed-color separation wherein the red toner is stuck to the non-image portion as aforementioned, and a rate of mixed-color may increase.
Particularly, when the image forming frequency increases in the state that image density of the red image, or a ratio of image portion relative to a whole image area is high, and image density of the black image is low, the red toner tends to mix largely into the second developing agent and the mixed toner can not be removed sufficiently, increasing the rate of mixed-color.
Meanwhile, in the case of the color toner which is charged in opposite polarity when mixed with the black toner, separation capacity also depends on the mixing ratio of the carrier and the black toner of the second developing agent and a stirring time and magnitude of the second developing agent, besides the aforesaid difference in physical properties between the color toner and the black toner, or the difference in electrification ranks. Here, the larger the mixed ratio is, the higher the separation capability becomes, and conversely, the separation capability disappears when the mixed ratio becomes 3% or less. Moreover, the separation capability reduces as the stirring time becomes longer, and the more the replenishing amount of the new black toner, the higher the separation capability.
In the case aforementioned, the amount of color toner mixed into the second developing agent depends on the conditions of the second development such as a developing process, physical properties of the carrier and setting condition, and the consumed amount of color toner. While, even when the color toner is mixed, in the case where the development by the black toner is effected, the color toner is stuck to the non-image portion and removed, so that when a large amount of black toner is used, the mixed amount is reduced gradually.
Furthermore, the first toner is mixed into the second developing agent only when the first developing unit is operated, and when only the second developing unit is operated during a standstill of the first developing unit, there is hardly any possibility that the first toner is mixed into the second developing agent. In other words, responsive to the used amount of first toner, the first toner mixed into the second developing agent increases. Accordingly, by detecting the consumed amount of the first toner and separating and recovering it in response to the detected result of detection, the consumption of the second toner can be prevented.